Class-AB/B amplifier with quiescent control circuit

ABSTRACT

Disclosed is a Class-AB/B amplifier comprising a first output stage including a first plurality of amplification devices and a second output stage including a second plurality of amplification devices. According to one embodiment, the first output stage operates when the Class-AB/B amplifier is in a quiescent state and the second output stage operates when the Class-AB/B amplifier is in an active state. The Class-AB/B amplifier also comprises a level shifting circuit that adjusts a control voltage of the second output stage, where the level shifting circuit is adapted to activate the second output stage when the Class-AB/B amplifier enters the active state. Embodiments of the Class-AB/B amplifier may include a level shifting circuit that implements either a fixed or signal-dependent level shift, and a quiescent control circuit that substantially eliminates any systematic offset arising from the active feedback circuit inside the replica bias circuit.

This is a continuation of application Ser. No. 12/807,403 filed09/03/2010.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of electronic circuitsand systems. More specifically, the present invention is in the field ofamplifier circuits and systems.

2. Background Art

Audio power amplifiers form an important part of many mobilecommunications devices, such as cellular telephones and MP3 players.Because these mobile communications devices are typicallybattery-operated, it is highly desirable that the audio power amplifiersimplemented in such devices consume low quiescent currents in order toextend the life of the battery. For example, a Class-A amplifiercontinuously consumes a large current even in the absence of an audiosignal, which makes it unattractive for battery-operated hand-helddevices. An alternative conventional amplifier, the Class-B amplifier,although designed so as not to consume any current when there is nosignal, suffers from large cross-over distortion. This cross-overdistortion may significantly degrade the output signal quality of aClass-B amplifier implemented to provide audio amplification.

A conventional approach to achieving a compromise between the relativelyhigh fidelity of the Class-A design and the efficiency achieved byClass-B amplifiers is the Class AB amplifier, which consume a smallcurrent in the quiescent state to improve cross-over distortion.However, typical Class-AB amplifiers employ large output-stagetransistors that are sized to handle peak load currents. This can createlarge variations in the output stage currents due to process mismatch,which may cause a stability problem if the output stage current issignificantly reduced. In addition, the large output stage transistorsand the small quiescent current reduce the headroom allocated to thefirst stage.

Thus, mobile communications devices may benefit from an audio amplifierthat combines the good cross-over distortion performance of Class-ABamplifiers and the low quiescent current of Class-B amplifiers, withoutincurring stability problems due to current variations or headroomlimitations. Accordingly, there is a need to overcome the drawbacks anddeficiencies in the art by providing a Class-AB/B amplifier and aquiescent control circuit for implementation with the Class-AB/Bamplifier.

SUMMARY OF THE INVENTION

The present application is directed to a Class-AB/B amplifier andquiescent control circuit for implementation with same, substantially asshown in and/or described in connection with at least one of thefigures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram that includes an amplification systemthat employs a Class-AB/B amplifier, according to one embodiment of thepresent invention.

FIG. 2 shows, in more detail, an amplification system that employs aClass-AB/B amplifier along with signal-independent level shiftingcircuitry, according to one embodiment of the present invention.

FIG. 3 depicts a signal-dependent level shifting circuit that issuitable for implementation with one embodiment of the presentinvention.

FIG. 4 shows an amplification system that employs a quiescent controlcircuit in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a Class-AB/B amplifier andquiescent control circuit suitable for implementation with theClass-AB/B amplifier. Although the present invention is described withrespect to specific embodiments, the principles of the invention, asdefined by the claims appended herein, can obviously be applied beyondthe specifically described embodiments of the invention describedherein. Moreover, in the description of the present invention, certaindetails have been left out in order not to obscure the inventive aspectsof the invention. The details left out are within the knowledge of aperson of ordinary skill in the art.

The drawings in the present application and their accompanying detaileddescription are directed to merely exemplary embodiments of theinvention. To maintain brevity, other embodiments of the invention,which use the principles of the present invention are not specificallydescribed in the present application and are not specificallyillustrated by the present drawings. It should be borne in mind that,unless noted otherwise, like or corresponding elements among the figuresare indicated by like or corresponding reference numerals. Moreover, thedrawings and illustrations in the present application are generally notto scale, and are not intended to correspond to actual relativedimensions.

Power efficient amplifiers form an integral part of many mobilecommunications devices like cellular telephones. Class-B amplifiers thatconsume no current in the absence of the signal, and Class-AB amplifiersthat exhibit better cross-over distortion each present potentiallydesirable configurations to amplify audio signals, for example. However,despite potential efficiencies over Class-A amplifiers, conventionalClass-AB and Class-B amplifiers have their own shortcomings. Forinstance, conventional Class-B amplifiers often have a cross-overdistortion that can severely degrade output signal quality when theoutput stage devices of the amplifier are activated. Moreover,conventional Class-AB amplifiers typically require large output stagedevices that must be sized to handle peak load currents. These largeoutput stage devices present a large parasitic capacitance, which cancause the amplifier to be unstable. To avoid those stability problems,the amplifier must be redesigned for reduced bandwidth. In addition, thelarge output stage device can undesirably limit the headroom allocatedfor the first stage in a conventional Class-AB amplifier. Moreover, in aquiescent state, the large output stage devices amplify the variation inquiescent current arising from random process mismatch among the smallbiasing devices implemented in the quiescent control circuit, which canfurther destabilize the amplifier. It is therefore desirable to have thehigh power efficiency of conventional Class-AB and Class-B amplifierswithout the drawbacks of these conventional amplifiers.

FIG. 1 illustrates a block diagram that includes amplification system100, according to one embodiment of the present invention, capable ofovercoming the drawbacks and deficiencies associated with theconventional art. Amplification system 100 may be implemented as anaudio amplifier in a mobile communications device, such as a cellulartelephone. As shown in FIG. 1, amplification system 100 may comprisereplica bias circuit 110 and Class-AB/B amplifier 150. Positive supplyvoltage 106 and negative supply voltage 108 may serve as supply voltagesto replica bias circuit 110 and Class-AB/B amplifier 150. Each ofpositive supply voltage 106 and negative supply voltage 108 maycorrespond to a fixed voltage source generated from the main supplyusing a linear or switched voltage regulator, for example.

Replica bias circuit 110 may supply control voltages to the controlterminals of individual devices that lie within Class-AB/B amplifier150. Replica bias circuit 110 may also maintain stable quiescentcurrents in Class-AB/B amplifier 150 when Class-AB/B amplifier 150 is ina quiescent state.

Class-AB/B amplifier 150 may include a novel configuration of circuitblocks such as first output stage 160, level shifting circuit 170, andsecond output stage 190. Operationally, Class-AB/B amplifier 150 mayamplify an input signal at input terminal 102 to provide an outputsignal at output terminal 104.

To illustrate the functional blocks in FIG. 1 in more detail, FIG. 2shows amplification system 200 according to one embodiment of thepresent invention. As shown, amplification system 200 may includereplica bias circuit 210 and Class-AB/B amplifier 250, each supplied bypositive supply voltage 206 and negative supply voltage 208. Class-AB/Bamplifier 250 may include first output stage 260, level shifting circuit270, and second output stage 290, and be configured to amplify an inputsignal at input terminal 202 to provide an output signal at outputterminal 204. Replica bias circuit 210, positive supply voltage 206,negative supply voltage 208, Class-AB/B amplifier 250 including firstoutput stage 260, level shifting circuit 270, and second output stage290, input terminal 202, and output terminal 204 correspond respectivelyto replica bias circuit 110, positive supply voltage 106, negativesupply voltage 108, Class-AB/B amplifier 150 including first outputstage 160, level shifting circuit 170, and second output stage 190,input terminal 102, and output terminal 104, in FIG. 1.

Class-AB/B amplifier 250 may include input current source 258, firstbiasing device 252 a, second biasing device 252 b, and input device 256.Any of devices 252 a, 252 b, and 256 may be field-effect transistors(FETs), such as metal-oxide-semiconductor FETs (MOSFETs), for example.Biasing devices 252 a and 252 b may receive control voltages fromreplica bias circuit 210 at first bias control terminals 253 a andsecond bias control terminal 253 b, respectively. Furthermore, biasingdevices 252 a and 252 b may supply bias voltages to first output stage260 at terminals 254 a and 254 b, respectively. Input device 256 mayprovide a value corresponding to an amplified version of the inputsignal at input terminal 202 to first output stage 260. In oneembodiment, first biasing device 252 a may comprise a p-channel MOSFET(PMOS device), while second biasing device 252 b and input device 256may each be an n-channel MOSFET (NMOS device).

According to the embodiment shown in FIG. 2, first output stage 260 ofClass-AB/B amplifier 250 includes a first plurality of amplificationdevices, such as first output-stage amplification device 262 and firstoutput-stage amplification device 264. First output stage 260 mayoperate when Class-AB/B amplifier 250 is in a quiescent state, wherein aquiescent state is defined as a state during which Class-AB/B amplifier250 does not drive substantial signal current to or from output terminal204. First-stage amplification device 262 may be a PMOS device andfirst-stage amplification device 264 may be an NMOS device, for example.

First-stage amplification devices 262 and 264 may be sized to carry thequiescent current of Class-AB/B amplifier 250, as well as a smallportion of the load currents. For example, first-stage amplificationdevices 262 and 264 may be sized substantially smaller than the largeramplification devices implemented as second-stage amplification devices292 and 294. (Second-stage amplification devices 292 and 294 arediscussed more thoroughly below.) Advantageously, the relatively smallsizes of first-stage amplification devices 262 and 264 reduce theeffects of quiescent current variations on amplifier output. Therelatively small sizes of first-stage amplification devices 262 and 264also increase signal headroom at nodes 254 a and 254 b in Class-AB/Bamplifier 250.

Class-AB/B amplifier 250 may further comprise second output stage 290including a second plurality of amplification devices, such assecond-stage amplification device 292 and second-stage amplificationdevice 294. Second output stage 290 may operate only when Class-AB/Bamplifier 250 is in an active state, wherein an active state is definedas a state during which Class-AB/B amplifier 250 drives signal currentsthat are substantially larger than the quiescent current to and fromoutput terminal 204.

Second stage amplification device 292 may be a PMOS device andsecond-stage amplification device 294 may be an NMOS device, forexample. Second-stage amplification devices 292 and 294 may be sized tocarry a majority of the load currents. For example, second-stageamplification devices 292 and 294 may be sized substantially larger thanfirst-stage amplification devices 262 and 264. The relatively large sizeof second-stage amplification devices 292 and 294 may ensure that largeoutput devices efficiently carry a majority of the load currents thatflow through Class-AB/B amplifier 250.

To deactivate these larger output stage transistors in second outputstage 290 in the quiescent state, e.g., second-stage amplificationdevices 292 and 294, Class-AB/B amplifier 250 may internally includelevel shifting circuit 270 to adjust a control voltage of second outputstage 290. Level shifting circuit 270 may perform a first level shift ata control terminal, such as a gate terminal, of second-stageamplification device 292, and a second level shift at a controlterminal, such as a gate terminal, of second-stage amplification device294. By selectively activating second-stage amplification devices 292and 294 only in the presence of a load current, level shifting circuit270 may activate second output stage 290 when Class-AB/B amplifier 250is in an active state.

In one embodiment, level shifting circuit 270 may implement a fixedlevel shift that does not depend on the value of the input signal. Forexample, level shifting circuit 270 may include first operationalamplifier 272, first feedback resistor 274, first current source 276,and ground terminal 278. Level shifting circuit 270 may include a firstfeedback system comprising first current source 276 coupled to thenegative input of first operational amplifier 272, and first feedbackresistor 274 coupling the output of first operational amplifier 272 toits negative input, where first current source 276 and first feedbackresistor 274 are embedded with first operational amplifier to providethe first feedback system. Also, a positive input of first operationalamplifier 272 may be coupled to terminal 254 a. In one embodiment, firstoperational amplifier 272 may be a unity gain buffer. Moreover, thevalue of first feedback resistor 274 and current source 276 maydetermine the amount of a first level shift up that level shiftingcircuit 270 provides.

Level shifting circuit 270 may further include second operationalamplifier 282, second feedback resistor 284, second current source 286,and positive supply voltage 206. Level shifting circuit 270 may includea second feedback system comprising second current source 286 coupled tothe negative input of second operational amplifier 282, and secondfeedback resistor 284 coupling the output of second operationalamplifier 282 to its negative input, where second current source 286 andsecond feedback resistor 284 are embedded with second operationalamplifier 282 to provide the second feedback system. A positive input ofsecond operational amplifier 282 may be coupled to terminal 254 b. Inone embodiment, second operational amplifier 282 may be a unity gainbuffer. Moreover, the value of second feedback resistor 284 and currentsource 286 may determine the amount of a second level shift that levelshifting circuit 270 provides.

The level shifting implemented using resistor 274 and current source 276for PMOS transistor 292, and resistor 284 and current source 286 forNMOS transistor 294 is embedded into the feedback systems of feedbackamplifiers 272 and 284 respectively. This has at least two benefits.First the additional poles formed by resistor 274 and the gatecapacitance of PMOS transistor 292, and by resistor 284 and the gatecapacitance of NMOS transistor 294 is now pushed to higher frequencies,which helps to maintain the stability of Class-AB/B amplifier 250.Second, unity-gain feedback amplifiers 272 and 282 buffer the gatecapacitance of large second-stage amplification devices 292 and 294 andwiden the bandwidth of Class-AB/B amplifier 250.

Another embodiment of the present invention may employ asignal-dependent level shifting circuit to vary the control voltage ofsecond output stage 290 based on the value of an input signal. Onebenefit of this approach is that the level shifting reduces to zero atthe peaks of the signal. This helps increase the current drivecapability of output devices 292 and 204, thereby making it possible toimplement designs which have reduced size transistors in the secondoutput stage 290, but which also maintain sufficient drive strength toachieve a given current drive requirement. One embodiment of such animplementation is shown in FIG. 3.

Turning to amplification system 300 in FIG. 3, an example embodiment ofthe present invention may include signal-dependent level shiftingcircuit 370, which is coupled to first-stage amplification device 362 atterminal 354, and which provides a signal-dependent level shift forsecond-stage amplification device 392. As shown in FIG. 3,signal-dependent level shifting circuit 370 is only intended for levelshifting up signal 354 going to the gate of PMOS transistor 392.However, it should be understood that a similar circuit could beemployed for level shifting down the gate of an NMOS transistor, such asNMOS transistor 294 in FIG. 2, as for example.

According to the embodiment shown in FIG. 3, signal-dependent levelshifting circuit 370 may comprise a scaling circuit to generate adependent voltage that is proportional to the input signal. For example,signal-dependent level shifting circuit 370 may comprise scaling device398 and scaling resistor 396, which is coupled to ground terminal 378.Scaling device 398 may be a transistor, such as a MOSFET, and may besized to handle a current that is proportional to the current flowingthrough first-stage amplification device 362. The current throughscaling resistor 396 may create a dependent voltage across scalingresistor 396 that is, in turn, proportional to the current flowingthrough first-stage amplification device 362.

Signal-dependent level shifting circuit 370 may also comprise acomparison circuit, for example, a comparison circuit comprising firsttransconductance block 394 a and second transconductance block 394 b,that generates a compared signal based on a comparison of the dependentvoltage to a reference voltage. First transconductance block 394 a mayhave a positive terminal coupled to scaling resistor 396, and a negativeterminal coupled to reference voltage source 388. Secondtransconductance block 394 b may have a positive terminal coupled toreference voltage source 388, and a negative terminal coupled to scalingresistor 396. The resulting compared signal may be detectable at theoutputs of transconductance blocks 394 a and 394 b.

Signal-dependent level shifting circuit 370 may further comprise anadjustment circuit that adjusts the control voltage of the second outputstage based on a value of the compared signal. For example,signal-dependent level shifting circuit 370 may comprise operationalamplifier 372, feedback resistor 374, first current source 376 a, andsecond current source 376 b. A negative input of operational amplifier372 may be coupled to feedback resistor 374 and first current source376, which is also coupled to ground terminal 378. A positive input ofoperational amplifier 372 may be coupled to terminal 354. In oneembodiment, operational amplifier 372 may be a unity gain buffer, andthe value of feedback resistor 374 and the signal level may determinethe amount of adjustment of the control voltage to large amplificationdevice 392. Moreover, the signal dependency may be implemented byselecting the current, resistance, and transconductance values of firstcurrent source 376 a, feedback resistor 374, and transconductance blocks394 a and 394 b in such a way that level shifting occurs only for smallsignal levels and reduces to a small positive value or a negative valuefor large signals. Thus, for small signals in which the current in firststage amplification device 362 is small, the voltage at the gate oflarge second-stage amplification device 392 is smaller than the voltageat terminal 354. For large signals in which the current in first stageamplification device 362 is large, the voltage at the gate of largesecond-stage amplification device 392 is equal or larger than thevoltage at terminal 354.

An embodiment of the present invention may also incorporate a quiescentcontrol circuit into a replica bias circuit to provide an accuratecontrol for the output-stage current, which may be necessary in order tomaintain the stability of Class-AB/B amplifier 250. Turning toamplification system 400 in FIG. 4, an embodiment of the presentinvention may include amplifier 450, which may be a Class-AB orClass-AB/B amplifier, for example, and replica bias circuit 410.

Amplifier 450 is shown in FIG. 4 in a Class-AB implementation comprisinginput device 456, first amplification device 462, second amplificationdevice 464, first biasing device 452 a, and second biasing device 452 b.Any of devices 456, 462, 464, 452 a, and 452 b may be a MOSFET, forexample. First biasing device 452 a and second biasing device 452 b maycouple Class-AB amplifier 450 to replica bias circuit 410 at firstbiasing terminal 453 a and second biasing terminal 453 b, respectively.

In this embodiment, replica bias circuit 410 may comprise first replicatransistor 422 coupled to first current source 412 a; second replicatransistor 436 and device 434 series coupled to second current source412 b; third replica transistor 438 coupled to third current source 412c; and fourth replica transistor 418 coupled to fourth current source412 d.

A quiescent control circuit within replica bias circuit 410 may includefirst active feedback circuit 411 to substantially match the currentdensity of second replica transistor 436 in replica bias circuit 410 tothe current density of first biasing device 452 a in amplifier 450.First active feedback circuit 411 may comprise device 430 andcompensation capacitor 432, which are shown coupled to first currentsource 412 a. First active feedback circuit 411 may further includedevice 424 and device 426 with a terminal, such as a source, coupled toa common node. Within first active feedback circuit 411, device 433 mayhave one terminal coupled to ground terminal 414 a, another terminalcoupled to fifth current source 412 e, and a third terminal coupled tocompensation capacitor 432. In operation, this configuration may ensurethat the quiescent current in first replica transistor 422 and secondreplica transistor 436 is set accurately as a scaled version of thecurrent flowing into first amplification device 462 and first biasingdevice 452 a, respectively.

Similarly, the quiescent control circuit within replica bias circuit 410may also include second active feedback circuit 413 to substantiallymatch the current density of third replica transistor 438 in replicabias circuit 410 to the current density of second biasing device 452 bin amplifier 450. Second active feedback circuit 413 may comprise device446 and compensation capacitor 440, which are shown coupled to fourthcurrent source 412 d. Second active feedback circuit 413 may furtherinclude devices 442 and 444, each with a terminal, such as a source,coupled to a common node. Within second active feedback circuit 413,device 435 may have one terminal coupled to ground terminal 414 b,another terminal coupled to sixth current source 412 f, and a thirdterminal coupled to compensation capacitor 440. In operation, secondactive feedback circuit 413 may ensure that the quiescent current infourth replica transistor 418 and third replica transistor 438 is setaccurately as a scaled version of the current flowing into secondamplification device 464 and second biasing device 452 b, respectively.

The quiescent control circuit within replica bias circuit 410 may sufferfrom a systematic offset problem if first level shifting device 428 andsecond level shilling device 448 are not included in replica biascircuit 410. First level shifting device 428 adds an internal levelshift to first active feedback circuit 411. If the current density offirst level shifting device 428 matches the current density of firstreplica transistor 422 and first amplification device 462, then thedesired value of internal level shift voltage is added to first activefeedback circuit 411 to substantially eliminate the systematic offset.First level shifting device 428 may be, for instance, a MOSFET coupledto first active feedback circuit 411. A first terminal of first levelshifting device 428 may be coupled to a supply voltage of replica biascircuit 410, such as positive supply voltage 406. First level shiftingdevice 428 may have second and third terminals, such as gate and drainterminals, coupled in common to first active feedback circuit 411, forexample. First level shifting device 428 may add a voltage shift to theoutput of first active feedback circuit 411, so that the systematicoffset of active feedback circuit 411 is substantially eliminated andthe quiescent current in first amplification device 462 is setappropriately.

The quiescent control circuit within replica bias circuit 410 mayinclude second level shifting device 448 to add an internal level shiftto second active feedback circuit 413. Second level shifting device 448may be, for example, a MOSFET coupled to second active feedback circuit413. If the current density of second level shifting device 448 matchesthe current density of fourth replica transistor 418 and of secondamplification device 464, then the desired value of internal level shiftvoltage is added to second active feedback circuit 415 to substantiallyeliminate the systematic offset. A first terminal of second levelshifting device 448 may be coupled to a supply voltage of replica biascircuit 410, such as negative supply voltage 408. Second level shiftingdevice 448 may have second and third terminals, such as gate and drainterminals, coupled in common to second active feedback circuit 413.Second level shifting device 448 may add a voltage offset to the outputof second active feedback circuit 413, so that the systematic offset ofactive feedback circuit 413 is substantially eliminated and thequiescent current in second amplification device 464 is setappropriately.

Unlike conventional Class-A amplifiers, a Class-AB/B amplifier accordingto embodiments of the present invention is characterized by high powerefficiency. For instance, unlike a conventional Class-A amplifier, theexample Class-AB/B amplifiers disclosed herein may deactivate outputstage devices during a portion of the span of an input signal andmaintain a high operating efficiency. Moreover, the Class-AB/B amplifierembodiment disclosed by the present application also possesses numerousbenefits over conventional Class-AB and conventional Class-B amplifiers.More specifically, a small and stable quiescent current may flow throughthe first output stage devices in presently disclosed Class-AB/Bamplifier, ultimately limiting significant cross-over distortion. Byemploying signal-dependent level shifting, embodiments of the presentinvention can also be implemented to allow output stage devices to beoptimally sized to handle peak load currents.

From the above description, it is manifest that various techniques canbe used for implementing the concepts of the present invention withoutdeparting from its scope. Moreover, while the invention has beendescribed with specific reference to certain embodiments, a person ofordinary skill in the art would recognize that changes could be made inform and detail without departing from the spirit and the scope of theinvention. The described embodiments are to be considered in allrespects as illustrative and not restrictive. It should also beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of many rearrangements,modifications, and substitutions without departing from the scope of theinvention.

The invention claimed is:
 1. A Class-AB/B amplifier comprising: a firstoutput stage including a first amplification device, said first outputstage operating when said Class-AB/B amplifier is in a quiescent state;a second output stage including a second amplification device; a levelshifting circuit including at least one operational amplifier andconfigured to adjust a control voltage of said second output stage. 2.The Class-AB/B amplifier of claim 1, wherein said level shifting circuitimplements a fixed level shift.
 3. The Class-ABB amplifier of claim 1,wherein said level shifting circuit further includes at least onefeedback system.
 4. The Class-ABB amplifier of claim 1, wherein said atleast one operational amplifier is a unity gain buffer.
 5. TheClass-AB/B amplifier of claim 4, wherein a level shift provided by saidlevel shifting circuit is determined by a current source and a resistorcoupled to said unity gain buffer.
 6. The Class-AB/B amplifier of claim1, wherein said first amplification device comprises at least one FET.7. The Class-AB/B amplifier of claim 1, wherein said secondamplification device comprises at least one FET.
 8. The Class-AB/Bamplifier of claim 1, wherein said Class-AB/B amplifier is adapted foruse as an audio amplifier.
 9. The Class-AB/B amplifier of claim 1,wherein said Class-AB/B amplifier is implemented in a mobilecommunications device.
 10. The Class-AB/B amplifier of claim 1 furthercomprising a quiescent control circuit to provide a control current forsaid Class-AB/B amplifier.
 11. The Class-AB/B amplifier of claim 1,wherein said level shifting circuit comprises a signal-dependent levelshifting circuit to vary said control voltage of said second outputstage based on a value of an input signal of said Class-AB/B amplifier.12. The Class-ABB amplifier of claim 11, wherein said signal-dependentlevel shifting circuit comprises a scaling circuit creating a dependentvoltage proportional to said input signal.
 13. The Class-AB/B amplifierof claim 12, wherein said signal-dependent level shifting circuitfurther comprises a comparison circuit generating a compared signalbased on a comparison of said dependent voltage to a reference voltage.14. The Class-AB/B amplifier of claim 13, wherein said signal-dependentlevel shifting circuit further comprises an adjustment circuit adjustingsaid control voltage based on a value of said compared signal.
 15. TheClass-AB/B amplifier of claim 11, wherein said Class-AB/B amplifier isadapted for use as an audio amplifier.
 16. The Class-AB/B amplifier ofclaim 11, wherein said Class-AB/B amplifier is implemented in a mobilecommunications device.
 17. The Class-AB/B amplifier of claim 11 furthercomprising a quiescent control circuit to provide a control current forsaid Class-AB/B amplifier.